You might want to consider giffgaff - they use the O2 network in the UK and in the last couple of days announced unlimited (truly unlimited) data usage for £10/month, rising to £35/month for truly unlimited text/voice calls/data.

It's a PAYG network, but they have "auto-topups" for when your credit drops below £3 - you set the amount to automatically top-up, between £10 and £30. Might work out to be more cost effective than a traditional SIM-only contract, without the risk of running out of credit, and you can cane the internet no worries...

http://www.whatmobile.net/News/gener...d_bundles.html